Antenna systems have near-field and far-field radiation regions. The near field is a region near an antenna where the angular field distribution depends upon the distance from the antenna. The near field is generally within a small number of wavelengths from the antenna and is characterized by a high concentration of energy and energy storage in non-radiating fields. In contrast, the far field is the region outside the near field, where the angular distributions of the fields are essentially independent of the distance from the antenna. Generally, the far-field region is established at a distance of greater than D2/λ from the antenna, where D is an overall dimension of the antenna that is large compared to wavelength λ. The far-field region is where radiation from the antenna is said to occur.
Some Radio Frequency Identification (RFID) systems, for example, use near fields for communications between the RFID tag and the RFID interrogator, and the energy stored in the near fields provides the power to drive a microchip imbedded in a passive RFID transponder tag. RFID systems are typically wireless, non-contact systems that use radio frequency electromagnetic fields to transfer information from an RFID card or tag to a reader for the purposes of automatic identification and/or tracking. Antennas used to create and exploit the energy in their near-field are found useful in RFID, nuclear magnetic resonance (NMR), quadrupole resonance (QR) and other applications. Used in this manner, these antennas may commonly be referred to as sensor probes.
At least some known RFID and some explosive detection systems use loop-type radiators for interrogator antennas, for example, an antenna consisting of a figure-eight shaped conductor to effect a reduction in the creation or reception of energy in their far-field regions. That is, loop antenna systems can be designed such that the coupling between the antenna and its nearby surroundings is relatively high, whereas the coupling between the antenna and its distant surroundings is minimized. By using two or more loops in combination, where the loops have a specific size and geometry, the magnitude of the current within the loops and the direction of the currents generated fields work to cancel each other out in the far-field region (that is, the sum of the fields created from each of the antenna loops is close to zero.)
One application for near field probes (including those using loop-type radiators) is in a detection system used to exploit a material's Nuclear Quadrupole Resonance (NQR), where NQR is a radio frequency (RF) magnetic spectroscopic technique that has been used to create a system to detect and identify a wide range of materials based on detection of the resonances associated with their quadrupolar nuclei. The NQR response signal provides a unique signature of the material of interest, where the detected electrical signal indicates the presence of quadrupolar nuclei. Exemplary uses for NQR include (but are not limited to), screening of airline baggage, parcel screening, detection of drugs/narcotics, and detection of explosives, such as detection of buried Improvised Explosives Devices (IED), and/or landmine detection. The particulars of the operation of several exemplary NQR detection systems are discussed in U.S. Pat. Nos. 6,777,937, 6,194,898, and 7,049,814, each of which is hereby incorporated by reference in its entirety.